(a) Field of the Invention
The present invention relates to a plasma display panel (PDP), and more particularly, to a plasma display panel having a structure preventing the reflection of external light to improve screen contrast.
(b) Description of the Related Art
A PDP is typically a display device in which vacuum ultraviolet rays generated by the discharge of gas occurring in discharge cells excite phosphors to realize predetermined images. As a result of the high resolution possible with PDPs (even with large screen sizes), many believe that they will become a major, next generation flat panel display configuration.
In a conventional PDP, with reference to FIG. 24, address electrodes 101 are formed along one direction (direction X in the drawing) on rear substrate 100. Dielectric layer 103 is formed over an entire surface of rear substrate 100 on which address electrodes 101 are located such that dielectric layer 103 covers address electrodes 101. Barrier ribs 105 are formed on dielectric layer 103 in a striped pattern and at locations corresponding to between address electrodes 101. Formed between barrier ribs 105 are red, green, and blue phosphor layers 107.
Formed on a surface of front substrate 110 facing rear substrate 100 are discharge sustain electrodes 112, 113 realized through a pair of transparent electrodes and bus electrodes 113. Discharge sustain electrodes 112, 113 are arranged in a direction substantially perpendicular to address electrodes 101 of rear substrate 100 (direction Y). Dielectric layer 116 is formed over an entire surface of front substrate 110 on which discharge sustain electrodes 112, 113 are formed such that dielectric layer 116 covers discharge sustain electrodes 114. MgO protection layer 118 is formed covering entire dielectric layer 116.
Areas between where address electrodes 101 of rear substrate 100 and discharge sustain electrodes 112, 113 of front substrate 110 intersect become areas that form discharge cells. Each of the discharge cells are filled with discharge gas.
An address voltage Va is applied between address electrodes 101 and one of discharge sustain electrodes 112, 113 to perform address discharge and thereby select discharge cells in which illumination is to occur, then a sustain voltage Vs is applied between a pair of the discharge sustain electrodes 112, 113 to perform sustain discharge. Vacuum ultraviolet rays (VUV) generated at this time excite corresponding phosphor layers such that visible light is emitted through transparent front substrate 110 to realize the display of images.
The PDP operating in this manner has a bright room contrast and a dark room contrast to a level exhibiting a contrast ratio. Bright room contrast refers to the contrast when a light source of 150 lux or greater exists to the exterior of the panel and the PDP receives the affect of the external light. Dark room contrast refers to the contrast when a light source of 21 lux or less exists to the exterior of the panel and the PDP receives no substantial affect of the external light.
In conventional PDPs, front substrate 110 is made of a transparent glass material such that the reflection of external light is unavoidable. The reflection of external light occurs when light from outside the panel passes through front substrate 110, reaches the discharge cells, and is reflected on phosphor layers 107 or dielectric layer 116. External light also reflects directly on an outer surface of front substrate 110.
In the case where external light passes through front substrate 110 to be reflected on either phosphor layers 107 or dielectric layer 116, the brightness of black display is increased. This reduces the dark room contrast of the screen. When external light is reflected directly from the outer surface of front substrate 116, part of the screen is shielded and therefore cannot be seen. This causes a decrease in the bright room contrast of the screen.
Accordingly, a light shielding film is formed between the discharge sustain electrodes 112, 113 of the conventional PDP such that light entering through front substrate 110 is blocked and prevented from being reflected. This is a common configuration used in PDPs. U.S. Pat. Nos. 5,952,782 and 6,200,182 disclose PDPs using such light shielding films between the front substrate and the phosphor layers.
However, with the mounting of light shielding films on the inner surface of the front substrate and therefore adjacent to areas of discharge, the material in the light shielding films used to block light negatively affects the discharge operation so that discharge does not occur normally. Further, the light shielding films are unable to prevent reflection from the outer surface of the front substrate. This may cause problems (i.e., significant reflection) when the PDP is placed in a room using fluorescent lights or other such high-intensity lighting, thereby being unable to prevent a reduction in bright room contrast.
Color characteristics of red, green, and blue phosphor layers determine the color temperature of the screen. The phosphors of these different color layers used in conventional systems have differing phosphor efficiencies and therefore varying brightness ratios. Accordingly, in order to improve color temperature, it is necessary to compensate for the phosphor with the lowest brightness ratio among these three colors of phosphors.
The typical method used to perform such color compensation in conventional PDPs is to perform gamma compensation so that peak values for the different colors are reduced. This is performed prior to digitizing analog image signals for the colors that do not have the lowest brightness ratios, for example, the red and green colors (assuming for the sake of this example that blue has the lowest brightness ratio). Therefore, the number of sustain pulses, which indicate maximum brightnesses of red and green, is reduced to below the number for blue. Further, the discharge cells containing the phosphor layers of the color exhibiting the lowest brightness ratio are made the largest, while the volumes for the discharge cells containing the phosphor layer of the other two colors are reduced in size. This further improves color temperature.
However, in the method utilizing gamma compensation described above, not all 255 sustain pulses needed for maximum green and red brightness are used. As a result, for images that gradually become bright or dark, green and red colors in the images realize such changes in increments and not in a gradual manner. Further, with the use of discharge cells of differing sizes, the likelihood of mis-discharge occurring increases, and a voltage margin, needed for stable driving, decreases.